Figure 3. Haem conformation in A. suum cytochrome b₅.

(A) Two haem conformations about the α,γ-meso axis are denoted as isomers A and B. The haem-Fe atom is bound to His³⁸ and His⁶² from the front and back sides respectively. Three residues, Ile²², Met³¹ and Asn⁴⁶, which are in close contact with the haem group, are shown in the respective Figure. (B) Stereo view of an F_o-F_c omit map for haem at 1.8 Å resolution. The haem group and residues within 10 Å of the haem are excluded from structure factor calculations and difference electron density is contoured at the 3σ level. Isomer B, the predominant haem form of A. suum cytochrome b₅, is drawn as a ball-and-stick model.